简单的是/否循环重复/结束-MIPS组装（MARS）

Question

bI'm not so sure why it's not reading the input and deciding to repeat or end.. Here is my code, help would be appreciated!

.data  
again:
    .asciiz "Again (y or n)? "  
answer:
    .asciiz " "  

.text  
.globl main  
  main:
    li  $v0, 4  
    la  $a0, again  
    syscall  

    la  $s4, answer
    jal get 

    beq $v0, 'y', main
    beq $v0, 'Y', main

    li  $v0, 10 
    syscall 

  get:  
    li  $v0, 12  
    li      $a1, 2  
    syscall  
    jr  $ra

Answer 1

Consider this:

.data
again:
    .asciiz "Again (y or n)? "  
answer:
    .space 256

.text  
.globl main  
  main:
    li  $v0, 4  
    la  $a0, again  
    syscall  

    la  $a0, answer
    li  $a1, 3
    li  $v0, 8
    syscall

    lb  $t4, 0($a0)

    beq $t4, 'y', main
    beq $t4, 'Y', main

    li  $v0, 10 
    syscall 

Firstly, in your code, you seem to misunderstand the way the syscalls work and how they ought to interact with functions ( some docs here ). Because your get routine basically just called a syscall, I took it out as adding a routine for something that basic is probably adding complexity rather than reducing it.

Next, the main problem of your code was a misunderstanding of the way input buffering works. In your code, you allocated exactly 2 bytes of space for answer , and then use syscalls to get no more than 2 bytes at a time. This will not work for stdin , because on most systems, stdin is line buffered, meaning that the user must press ENTER in order to flush the stream. This means that when the user types 'y' , a syscall actually returns "y\\n\\0" .

To fix this I expanded the syscall to read 3 characters and answer to store up to 256. Expanding this to be safe for any size is an excersize left to the reader.